The present invention relates generally to an apparatus and method for detecting light and, more particularly, to a mercuric iodide light detector or "photodetector" which is useful with a scintillator to detect ionizing radiation such as gamma rays, neutrons and charged particles.
Commercially available photomultiplier (PM) tubes have been used to detect and amplify light in a variety of circumstances, including radiation detection. However, radiation detectors using PM tubes for light detection have certain inherent disadvantages. PM tubes are generally rather bulky and cannot be miniaturized beyond a practical minimum size because high voltage dynodes within them must be suitably spaced from each other. The tubes also dissipate substantial power, drawing currents on the order of milliamperes at voltages greater than one kilovolt, and must be used with a power supply capable of delivering the required currents very rapidly.
The relatively large size and high power requirements of PM tubes are dictated by the physical characteristics of the tubes as electron multiplication devices, and limit their effectiveness in many situations. In tomography systems, for example, the physical size of PM tubes severely limits spatial resolution. The high power requirements also limit the resolution of tomography systems and place severe constraints on the design of portable, self-contained radiation detecting instruments.
PM tubes also must be shielded in many applications from magnetic fields, even the naturally occurring magnetic field of the earth, and possess substantially lower quantum efficiency than some solid states devices. The photocathode quantum efficiency of a PM tube is usually less than 30%, as compared with a potential quantum efficiency of close to 100% for solid state photodetectors.
Various solid state photodiodes have been used as light detectors. Existing types include silicon, gallium arsenide and indium phosphide. Such devices generally have objectionably high noise for applications involving low level input signals, due primarily to excessive currents or capacitances.
Silicon photodiodes have been described by Tuzzolino et al. "Photoeffects in Silicon Surface-Barrier Diodes", Journal of Applied Physics, Volume 33, No. 1, January 1962. Such diodes commonly have thin transparent contacts of gold or other suitable conductive material and are operated in the reverse-bias mode. They are significantly smaller than PM tubes, however they have leakage currents so large as to limit their performance at room temperature. Silicon photodiodes are therefore inherently noisy in low signal level operation, making it difficult or impossible to obtain a meaningful signal over noise unless the diodes are cooled to very low temperatures.
Scintillation detectors have been formed by combining either PM tubes or silicon photodiodes with scintillators. Ionizing radiation enters the scintillator and interacts with it to produce light photons distributed around a characteristic frequency. This light output is detected by the PM tube or the silicon photodiode. Detectors using PM tubes and scintillators of either sodium iodide (thallium), cesium-iodide (thallium) or bismuth germanate (hereinafter NaI(Tl), CsI(Tl), and BGO, respectively) are available from the Harshaw Chemical Company of Solon, Ohio, as well as other commercial suppliers. They are useful to detect a wide variety of ionizing radiation, including gamma (.gamma.) radiation, neutron radiation and charged particle radiation.
Scintillation detectors are particularly useful in the field of positron emission tomography (PET) to trace the location and flow of positron-emitting pharmaceuticals within the human body. Information as to location is plotted by computer to yield an image of an organ or blood vessel into which the pharmaceuticals have passed. Positrons emitted by the pharmaceuticals combine almost instantaneously with an electron of the surrounding material to produce two quanta of gamma radiation which travel in opposite directions. Each quantum has an energy of 511 keV, half of the annihilation energy of the positron and electron. The gamma rays are detected by an array of scintillation detectors and the information is computer-analyzed to determine the location of annihilation and thus the location of the positron-emitting material. This technique is thoroughly discussed in Hoffman et al. "An Analysis of Some of the Physical Aspects of Positron Transaxial Tomography", Comput. Biol. Med., Volume 6, pp. 345-360, 1976.
In the technique of PET, the number of detectors and the proximity of the detectors to each other limits the resolution of the image obtainable by computer analysis. The relatively large size of PM tubes therefore severely limits image resolution, as discussed above. On the other hand, silicon photodiodes are unsatisfactory for use in the energy ranges encountered in PET. Such photodiodes generally require a signal of several MeV to yield an output signal above noise at room temperature.
Mercuric iodide has been used by the applicants herein as a detector of low energy x-rays, as described in Dabrowski et al., "Performance of Room Temperature Mercuric Iodide (HgI.sub.2) Detectors in the Ultralow-Energy X-Ray Region", IEEE Transactions on Nuclear Science, Volume NS-28, No. 1, February 1981. In this context, the wide band gap (2.1 eV) and resultant high electrical resistivity of HgI.sub.2 suppresses noise by limiting leakage current to approximately 10.sup.-12 amperes under normal operating conditions. In addition, good electron collection allows construction of a thick, low-capacitance detector. As a result, the signal-to-noise ratio of the detector is relatively high. When used in combination with low noise preamplifiers, including those of the pulsed-light feedback type developed by applicants and disclosed by Iwanczyk et al., "A Study of Low-Noise Preamplifier Systems for Use with Room Temperature Mercuric Iodide (HgI.sub.2) X-Ray Detectors", IEEE Transactions on Nuclear Science, Volume NS-28, No. 1, February 1981, a useable signal can be obtained over the noise.
In the above-referenced x-ray detectors, the contacts covering the HgI.sub.2 are designed for x-ray penetration and not to transmit light. Palladium films thicker than approximately 500 angstroms or thick carbon films have been used for this purpose. In photodetection, however, it is imperative that most of the light incident on the active contact area of the detector be transmitted to the interior of the detector. The thicker contacts used for x-rays are not acceptable. The problem of providing a light transmissive contact is compounded in the case of HgI.sub.2 by the fact that it tends to chemically react with many of the materials used to form contacts for silicon photodiodes and other solid state devices.
The photoresponsive properties of HgI.sub.2 were disclosed in Bube, "Opto-Electronic Properties of Mercuric Iodide", Physical Review, Volume 106, No. 4, 1957. The properties of HgI.sub.2 were investigated by Bube using localized graphite contacts on the material. The electrodes themselves were apparently not transparent and established a highly nonuniform potential distribution along the active portion of the material.